There are many different situations in which a system is monitored by a plurality of sensors. Often such sensors are part of different data collection devices, and monitor the same or different parameters of the system. In this situation each of the data collection devices may include its own clock controlling the sampling of the data signal it is monitoring. These clocks may be free running with respect to each other. Thus the output signals from the devices may not be synchronised and may be at widely differing rates.
For instance, it is normal when monitoring the condition of a patient to monitor a variety of physiological parameters such as the electrocardiogram (which can be multiple channel), blood pressure, respiration, oxygen saturation using pulse oximetry and temperature. Typically these are acquired by different data collection devices and all are acquired at different sampling rates. For example electrocardiograms (ECG) are typically collected at 256 Hz, pulse oximetry waveforms are typically acquired at 81.3 Hz, respiration waveforms at 64 Hz, temperature at 1 Hz and blood pressure once every 10 or 20 minutes. All of these vital signs are of clinical significance and are usually displayed so that medical staff can easily monitor the condition of the patient. However, because all are measured at different rates, and typically by different pieces of apparatus with respective system clocks within them, displaying the different parameters together in a concise and synchronised way is difficult.
In order to overcome the problem of synchronizing the different signals, one solution has been proposed which is to drive all of the different monitors by the same clock signal. However, this requires that all of the monitors are, in essence, integrated which is expensive and inflexible, and further this makes existing equipment redundant.
The display of the data is also rendered difficult because parameters such as the ECG trace vary on a fast timescale compared to parameters such as blood pressure (which is only measured every 10 to 20 minutes). Thus the timing of samples in an ECG trace needs to be accurately recorded. However, the timing of samples of the blood pressure can be of lower accuracy without the loss of clinical significance.
Similar problems arise in other systems, such as plant monitoring and control, e.g. of chemical processing plants, monitoring and control of machines, such as engines or vehicle systems.
According to the present invention there is provided a system for acquiring data from a plurality of data collection devices each monitoring a parameter and outputting a data signal at a respective sampling frequency based on respective system clocks, the system comprising                data processing means having:                    input means for receiving data signals from each of the plurality of data collection devices;            a master clock for providing a master clock signal; and            time stamping means for associating a time stamp derived from the master clock with each of the data signals.                        
Thus the invention allows data to be collected from a variety of different data collection devices, but the data samples are given a timestamp which is synchronised with a master clock.
The timestamp may have a higher resolution than the master clock. The master clock produces a new time value at regular intervals. The number of such intervals within a second is known as the tick-rate. The resolution on the time axis is the inverse of the tick-rate.
Preferably the time stamp associated with the samples is calculated in a different way depending on the sampling frequency of the data signal. For data signals (such as in a physiological environment the blood pressure or temperature) whose sampling frequency is below a predetermined threshold, each sample of the data is associated with a time stamp which is simply the value of the master clock signal at the time the data is given the timestamp. However for data signals whose sampling frequency is above the predetermined threshold (such as in a physiological environment the ECG, pulse oximetry or respiration waveforms) a first sample (or an appropriate sample in a first batch) of the data signal is associated with the value of the master clock signal at the time of time stamping, but subsequent samples are provided with an estimated time stamp. This may be based on a time interval calculated from the sampling frequency of the data collection device providing that signal (based on the known specifications of the data collecting device).
Preferably the estimate is periodically compared with the current value of the master clock to determine whether the difference between them is acceptable, or greater than a predetermined amount. If it is greater than the predetermined amount then the time stamp is corrected. Further, the time stamps of a contiguous set of samples preceding the current sample are also adjusted, for instance by adjusting them so that they are evenly spaced in time up to the current sample. The predetermined difference below which correction is regarded as unnecessary may be a multiple (between 5 and 50, for example) of the master clock's resolution and the predetermined threshold of sampling frequency may be less than or equal to the master clock frequency, preferably less than one fifth of the master clock frequency.
As well as adjusting the time stamps of the set of samples preceding the current sample, the manner in which the time stamp is estimated for future samples can be adjusted by adjusting the value of sampling interval used in the calculation. Thus by correcting that value it is hoped that the estimated time stamp will not diverge (or not diverge so quickly) from the value of the master clock. This adjustment can be achieved using a Kalman filter in which the value for the accuracy of the sampling interval is set in accordance with the time taken for the estimated time stamp to diverge significantly from the master clock.
In one embodiment for use in monitoring a physiological system (such as a patient) the system is suitable for receiving and displaying signals from an ECG monitor, oxygen saturation monitor, respiration monitor, blood pressure monitor and thermometer, or indeed any other transducer or monitor used for acquiring physiological data.
Preferably the system is based around a data processing device, which incorporates the master clock, the time stamping means and the display, and the system may be ruggedized so as to be easily portable without risk of damage.
To improve the clarity of the display the data may be displayed selectively on one of two different timescales which may be referred to as a short term continuous timescale, e.g. a “beat-to-beat” timescale in a physiological environment, as in which the time axis shows a short period of data in detail, e.g. a few seconds of data (typically from 1 to 60), and a “trend” timescale in which the time axis shows a longer section of data, e.g. a few hours of data (typically this may go from 1 minute to 1 day).
The parameters displayed and the manner of their display may be varied between the two types of display. For instance, on displaying data at the first timescale, data sampled at a low sampling frequency, can be displayed as a numerical value, rather than a continuous trace (which would have little meaning at this timescale given its much lower sampling frequency). On the other hand, in the “trend” timescale it may be useful still to display a single high frequency trace at the shorter timescale so that a continuous visual check of this trace can be maintained, even though the rest of the data is viewed over a long timescale. Preferably key values for the system, such as in the physiological environment the heart rate, blood pressure, oxygen saturation and temperature, are always displayed as numerical values alongside the traces in both display modes.
As a further improvement of the display the representations of the signals, namely the traces, may be scrolled with respect to the time axis as the data signals are received. This contrasts with the normal practice when displaying signals of refreshing the displayed trace repeatedly.
The invention provides a corresponding method of synchronizing data signals and the invention maybe embodied as a computer program comprising program code means for carrying out the method. The invention thus extends to a computer-readable storage medium carrying such a program.